1. Field of the Invention
The present invention relates to a heart-sound detecting apparatus which can determine a timing when aortic valve opens, a system for measuring a pre-ejection period by using a heart-sound detecting apparatus, and a system for obtaining information relating to a velocity at which a pulse wave propagates, by using a heart-sound detecting apparatus.
2. Related Art Statement
It is known that a first heart sound I, shown in FIG. 6A, has the following relationship with respective internal pressures of left ventricle, left atrium, and aorta of left half portion of the heart of a living person: When contraction of the ventricular muscle starts, the internal pressure of the left ventricle rapidly increases and, when the internal pressure of the left ventricle exceeds that of the left atrium, the mitral valve closes, which produces an initial portion of the first heart sound I. When the internal pressure of the left ventricle further increases and eventually exceeds that of the aorta, the aortic valve opens and ejection of blood starts so that blood flows from the left ventricle into the aorta. The opening of the aortic valve produces a remaining portion of the first heart sound I.
The sound produced by the opening of the aortic valve appears before the sound produced by the closing of the mitral valve disappears. Thus, in the first heart sound I, the sound resulting from the opening of the aortic valve overlaps the sound resulting from the closing of the mitral valve. In addition, the first heart sound I includes a sound resulting from the right half portion of the heart, such as a sound produced by the closing of the tricuspid valve that occurs substantially simultaneously with the closing of the mitral valve. Moreover, the first heart sound I includes internal noise produced in the body of the person. Thus, the first heart sound I is a complex sound and accordingly it has been difficult to determine, based on the first heart sound I, a timing when the aortic valve opens.
It is therefore an object of the present invention to provide a heart-sound detecting apparatus which can accurately determine a timing when aortic valve opens.
The above object has been achieved by the present invention. According to a first feature of the present invention, there is provided a heart-sound detecting apparatus, comprising a heart-sound microphone which detects at least one heart sound produced by a heart of a living subject and outputs a heart-sound signal representing the detected heart sound; a time-frequency analyzing means for analyzing, with respect to time and frequency, at least a portion of the heart-sound signal that includes a first heart sound I; and an aortic-valve-opening-timing determining means for determining a timing when an aortic valve of the heart opens, based on a time when a magnitude of the analyzed signal at a frequency higher than a frequency range of a main component present in an initial portion of the first heart-sound I is greater than a prescribed threshold value.
The sound resulting from the opening of the aortic valve follows the sound, resulting from the closing of the mitral valve and the tricuspid valve, as the main component present in the initial portion of the first heart sound I, and provides part of the remaining portion of the first heart sound I. Since the aortic-valve-opening-timing determining means determines a timing of opening of the aortic valve, based on a time when a magnitude of the analyzed signal at a frequency higher than a frequency range of the main component present in the initial portion of the first heart-sound I is greater than a prescribed threshold value, the determining means can accurately determine the timing.
According to a second feature of the present invention, there is provided a heart-sound detecting apparatus, comprising a heart-sound microphone which detects at least one heart sound produced by a heart of a living subject and outputs a heart-sound signal representing the detected heart sound; a time-frequency analyzing means for analyzing, with respect to time and frequency, at least a portion of the heart-sound signal that includes a first heart sound I; and an aortic-valve-opening-timing determining means for determining a timing when an aortic valve of the heart of the subject opens, based on a time when at least one magnitude of the analyzed signal at least one frequency in a frequency range of a main component present in an initial portion of the first heart-sound I is minimal.
The magnitude of the analyzed signal at a certain frequency in the frequency range of the main component present in the initial portion of the first heart-sound I first decreases, and then increases as the aortic valve opens. Since the aortic-valve-opening-timing determining means determines a timing of opening of the aortic valve, based on a time when at least one magnitude of the analyzed signal at least one frequency in the frequency range of the main component present in the initial portion of the first heart-sound I is minimal, the determining means can accurately determine the timing.
The heart-sound detecting apparatus according to the first or second feature may be employed as part of a pre-ejection-period measuring; system. According to a third feature of the present invention, there is provided a system for measuring a pre-ejection period between a timing when contraction of a heart of a living subject starts and a timing when blood is ejected from the heart to an aorta of the subject, the system comprising an electrocardiograph for detecting an electrocardiogram from the subject; a heart-sound detecting apparatus according to the first or second feature; and a pre-ejection-period determining means for determining, as the pre-ejection period, a time difference between a timing when a portion of the electrocardiogram indicative of excitation of a ventricular muscle of the heart is detected by the electrocardiograph, and the timing of opening of the aortic valve determined by the aortic-valve-opening-timing determining means of the heart-sound detecting apparatus.
In the present system, the aortic-valve-opening-timing determining means of the heart-sound detecting apparatus accurately determines a timing of opening of the aortic valve, and the pre-ejection-period determining means determines, as the pre-ejection period, a time difference between a timing when a portion of the electrocardiogram indicative of the excitation of the ventricular muscle of the heart is detected, and the timing of opening of the aortic valve. Thus, an accurate pre-ejection period is determined.
The heart-sound detecting apparatus according to the first or second feature may be employed as part of a pulse-wave-propagation-velocity-relating-information obtaining system. According to a fourth feature of the present invention, there is provided a system for obtaining information relating to a propagation velocity at which a pulse wave propagates along an artery of a living subject, the system comprising a heart-sound detecting apparatus according to the first or second feature; a pulse-wave detecting device which is adapted to be worn on the subject to detect the pulse wave which propagates along the artery of the subject; and a pulse-wave-propagation-velocity-relating-information obtaining means for obtaining the information based on the timing of opening of the aortic valve determined by the aortic-valve-opening-timing determining means of the heart-sound detecting apparatus, and a timing when a rising point of the pulse wave is detected by the pulse-wave detecting device.
In the present system, the aortic-valve-opening-timing determining means of the heart-sound detecting apparatus accurately determines a timing of opening of the aortic valve, and the pulse-wave-propagation-velocity-relating-information obtaining means obtains pulse-wave-propagation-velocity-relating information based on the timing of opening of the aortic valve accurately determined by the aortic-valve-opening-timing determining means, and a timing when a rising point of the pulse wave is detected by the pulse-wave detecting device. Since a time difference between the timing of opening of the aortic valve and the timing of detection of the rising point of the pulse wave does not include a pre-ejection period, the time difference may be used as a sort of pulse-wave-propagation-velocity-relating information.
In a conventional method, pulse-wave-propagation-velocity-relating information is obtained by, e.g., using an electrocardiograph or a heart-sound microphone to detect an electrocardiogram or at least one heart sound as a heartbeat-synchronous pulse wave from an upstream-side portion of an artery. In this case, a time when a portion (e.g., Q-wave, R-wave, or S-wave) of the electrocardiogram signal that represents excitation of the ventricular muscle is detected, or a time when a first heart sound I starts, may be used as a first reference time. However, a time difference between the first reference time and a second reference time detected by a pulse-wave detecting device from a downstream-side portion of the artery includes a pre-ejection period PEP between the time when the excitation of the ventricular muscle starts and the time when blood is ejected into the aorta. Hence, it is needed to subtract the pre-ejection period PEP from the above-indicated time difference so as to calculate a pulse-wave propagation time DT needed for the pulse wave to propagation from the heart to the pulse-wave detecting device. Since, however, it is difficult to measure a pre-ejection period PEP, a prescribed value is used as the pre-ejection period PEP in many cases. However, the pre-ejection period PEP can change depending upon the condition of the heart, the conventional method cannot obtain sufficiently accurate pulse-wave-propagation-velocity-relating information. Since the pre-ejection period PEP is changed by, e.g., preload, the period PEP is used as an index to evaluate the cardiac function of a patient.